1. Field of the Invention
The present invention generally relates to the field of semiconductor fabrication. More particularly, the invention relates to the fabrication of thin films in semiconductor devices.
2. Background Art
Bulk acoustic wave (BAW) structures, which can be used in frequency control or filtering applications, can include a piezoelectric layer sandwiched between upper and lower electrodes and an underlying acoustic mirror structure. The acoustic mirror structure, which can include a number of alternating dielectric and metal segments, can be used to trap acoustic energy in the piezoelectric layer by reflecting acoustic energy. However, as a result of metal layer patterning by conventional etch processes, the sidewalls of the metal segments in the acoustic mirror structure and the lower electrode can cause abrupt steps in the topography of the later deposited piezoelectric layer. The abrupt topographic steps can cause vertical displacements in the grain structure of the piezoelectric layer, which, in turn, can cause undesirable voids to form in the grain structure. Also, it can be very difficult to deposit thin interconnect metal segments that are smooth and reliable over the aforementioned abrupt steps in the topography, thereby undesirably affecting device reliability.
In a conventional approach for reducing the abrupt steps in topography caused by the sidewalls of metals segments in the acoustic mirror structure and the lower electrode, the acoustic mirror structure can be formed in a trench in the substrate underlying the BAW structure, thereby providing a smoother topography for piezoelectric layer formation. In the conventional approach discussed above, the lower electrode can also be formed in the trench to further reduce the abrupt steps in the topography of the BAW structure. However, burying the acoustic mirror structure can require very controlled etch steps, which are difficult to perform, and can also require chemical mechanical polishing (CMP) for surface smoothing, which is an expensive and difficult to control manufacturing process. As a result, the aforementioned conventional approach can undesirably increase manufacturing cost.